Molecular cloud formation with self-consistent thermal and chemical treatment of the gas

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Abstract

Understanding physical and chemical processes that guide the formation and evolution
of giant molecular clouds (GMCs) has important implications for the formation of stars.
GMCs dominantly consist of molecular hydrogen, but there are more than 200 chemical
species of various combinations of carbon, nitrogen and oxygen atoms. Together, these
species control the cooling ability with the thermal and dynamical evolution of the
gas cloud. In order to overcome the restrictions encountered by most previous models
of molecular cloud formation due to the complexity of chemical reaction networks
and its inclusion in hydrodynamical codes, we have implemented detailed treatment
of atomic/molecular cooling and hydrogen chemistry into state-of-art high resolution
hydrodynamical simulations. The main focus of our study is on the influence that
choosing between different cooling functions and turbulent driving has on the formation
and evolution of molecular gas. In that manner, we study the influence of the nature of
the turbulence on the formation of molecular hydrogen by examining both solenoidal
(divergence-free) and compressive (curl-free) turbulent driving. The obtained results
we use to test a simple prescription suggested by Gnedin et al. (2009) for modelling
the influence of unresolved density fluctuations on the H2 formation rate in largescale
simulations of the ISM. We also investigate the properties of the dense clumps
formed within our model of the molecular cloud formation in converging flows and
directly compare the results obtained using the simple, parametrized cooling function
introduced by Koyama & Inutsuka (2002) and used by a number of converging flows
studies with the results of the detailed calculation of the non-equilibrium chemistry
and thermal balance of the gas. Finally, we study C I and CO emission from molecular
clouds in comparison to their column densities and the total column density, as we
look for the way to trace the structure of the cloud.